The present invention generally relates to a system for mixing a gas into a solvent used in semiconductor manufacturing. More particularly, the invention relates to a system and process for variably elevating the levels of ozone gas dissolved in water used in semiconductor manufacturing.
In semiconductor manufacturing, ozone may be added to water-based solvents where it can serve the role of oxidant in certain manufacturing processes. Ozone gas is not readily dissolved in water, but when it does, it is chemically unstable and converts quickly to oxygen. Therefore, it is difficult to store ozonated water solutions, so mixing and use generally occur on-site with contemporaneous use of the solution in a process.
In semiconductor manufacturing, an ozonated solvent may be introduced at various stages into process vessels where, for example, a silicon wafer is being prepared, conditioned and modified to become the final semiconductor product. Generally, ozonated solvents are used in etching and stripping phases. More particularly, it is used in processes known in the art as piranha, organic strip, silicon oxide, passivation, or other film or process-free treatments.
It is difficult to obtain ozone levels in excess of 5 ppm in deionized water at 20.degree. C.-35.degree. C. degrees, which is a range of ambient temperatures for water supplied by some kinds of deionized water sources used in semiconductor fabrication. If ozone concentrations could be raised significantly above 5 parts per million (ppm) in an efficient, economic way, such a system would be a significant advancement in semiconductor processing which should pave the way for faster and more effective processing of semiconductor materials and lowered production costs.
Unfortunately conventional techniques for ozonating solvents do not result in optimal levels of dissolved ozone in the solvent. For example, in one conventional mixing system, ozone is introduced from an ozone generator through a diffuser into an ozone-water contact reaction tank for mechanical mixing with solvent. The diffuser forms the ozone gas into bubbles. The amount of ozone that can be dissolved in water depends on the total surface area of bubbles introduced into the water; the smaller the bubble diameter, the greater the potential surface area in a given volume of water. There are at least two significant limitations to generating bubbles of relatively small diameter: (1) the ozone generator must be run under power conditions that are economically undesirable; and (2) conventional diffusers do not form bubbles of small enough diameter to result in optimal dissolution of ozone in water.
U.S. Pat. No. 5,431,861 describes a system intended for producing elevated concentrations of ozone in water. In the system, raw material water is pumped into a reaction tank. Mixing of water and ozone is facilitated by a first injection nozzles that injects ozone gas as fine bubbles into the raw material water in the ozone reaction tank. A second injection nozzle creates a water jet that circulates water in the reaction tank. According to the '861 patent, the water must be kept above 00 .degree.C. and below 15.degree. C. to achieve elevated levels of ozone dissolution. Consequently, the reaction tank includes a refrigerator/heat exchanger to chill the solution.
Unfortunately, the system of the '861 patent appears to require relatively complicated ejectors, circulation means, and refrigeration means. These features are not desirable in the semiconductor industry because they are unduly complicated and expensive. They are also difficult to implement into existing semiconductor manufacturing systems and appear to be unduly difficult to operate and maintain.
Consequently, there is still a need in semiconductor manufacturing for an ozonization system that produces relatively high levels of dissolved ozone in solvent without complicated or expensive additions to existing fabrication equipment and conditions. Ideally, the system should be constructed from existing semiconductor manufacturing equipment. It should also act to elevate ozone levels in water at ambient temperatures so that cooling systems are not necessary.